1. Technical Field
This invention is related to the field of integrated circuit implementation, and more particularly to the implementation of clock signal monitoring circuits.
2. Description of the Related Art
Computing systems may include one or more systems-on-a-chip (SoCs), which may integrate a number of different functions, such as, graphics and audio processing, onto a single integrated circuit. With numerous functions included in a single integrated circuit, chip count may be kept low in mobile computing systems, such as smartphones and tablets, for example, which may result in reduced assembly costs, and a smaller form factor for such mobile computing systems.
Many of the functional blocks of an SoC may require a clock source to function, to communicate with other blocks in the SoC, and/or to communicate with other chips in the system. SoCs may have one or more clock inputs to receive a clock signal from elsewhere in the system to be used as a clock source inside the SoC. SoCs may include a crystal oscillator circuit that requires a crystal to be coupled to the SoC to generate a clock signal within the SoC. Some SoCs may have phase-locked-loop circuits (PLLs) or frequency-locked-loop circuits (FLLs) internally that may take the clock input from the system or from the crystal oscillator and multiply and/or divide the input to generate a clock signal with a given frequency to best match the needs of the application. In some embodiments, an SoC may generate a clock signal internally, which may be used directly or used as the input to an FLL or PLL.
Some system-on-a-chip (SoC) designs may be used in applications which may be targeted by a subset of users who attempt to gain unauthorized access into the system (commonly referred to as “hacking a system” or “hacking”). Some typical applications targeted for hacking include mobile phones, tablet computers, and video game systems.
A common method used to attack systems is to overdrive the clock input into one of the processing chips in the system. The intent is to force the chip into a logic error which may result in the chip entering a state from which the attacker may gain control over the execution of the processor. Once the attacker has whole or partial control of the chip, the attacker may be able to access information within the system, such as, e.g., security keys for accessing a cellular network, encryption key for data stored in the system, and the like.
A clock signal monitoring circuit may be used within a system for the purpose of detecting if the frequency of a clock signal is operating at a frequency higher than for which the system is designed. A clock signal monitoring circuit may sample an input clock signal to determine if the input clock is running above or below a predetermined frequency. Several embodiments of a clock signal monitoring circuit will be discussed below.